In this study, the following effects of mulberry root bark extracts (MRB) were examined: inhibition of the proliferation of the human colorectal cancer cells and cyclin D1 downregulation. MRB dose-dependently suppressed the growth of human colorectal cancer cells including HCT116, SW480 and LoVo. In addition, the proliferation of human breast cancer cells such as MDA-MB-231 and MCF-7 was inhibited by MRB treatment. However, MRB did not af- fect the proliferation of HepG-2 cells as a human hepatocellular carcinoma cell line.